Efficiency of Erbium-Doped Yttrium Aluminum Garnet Laser in Debonding Cemented Glass Fiber Posts: an in vitro Study

Statement of the Problem: The efficacy of erbium-doped yttrium aluminum garnet (Er; YAG) laser on the debonding properties of certain post materials has remained largely unexplored. Purpose: This study aimed to investigate the effect of Er; YAG laser irradiation on debonding of cemented glass fiber posts in root canal treated teeth. Materials and Method: In this in vitro study, forty root canal treated mandibular premolar teeth were used in this study. Glass fiber posts were bonded using Panavia F 2.0 cement in the root canal space, and samples were divided into two groups. In the test group, samples were exposed to laser radiation of 7W, 350mJ, frequency of 20Hz and discontinued washing spray. In the control group, samples were left untouched. In each group, samples were sectioned into 1.5mm thick slices from the coronal, middle, and apical thirds of the root (N=120). Tensile bond strengths were evaluated using the push-out test and the failure patterns were evaluated using scanning electron microscopy (SEM). To compare the laser and non-laser groups at each location, independent sample t test was applied, and to compare bond strength between the locations in each group, one-way ANOVA and Tukey’s HSD post hoc was applied. Results: A significant difference in tensile strength was observed between the laser-irradiated group and control group; tensile bond strength was much higher in the control group (p<0.001). The highest frequency of fractures was observed at the cement-dentin interface. Given the used parameters, complete debonding was not achieved in the laser-irradiated group. Conclusion: Laser radiation reduced the bond strength of glass fiber posts to resin cement without complete debonding.


Introduction
Teeth with insufficient residual coronal structure, caused by caries lesion, fracture, or extensive cavity preparation can be treated with various endodontic posts. One of the main functions of endodontic posts is to provide retention and stability for favorable coronal restoration [1]. Disadvantages associated with metal posts have led to the development of different esthetic posts among which prefabricated posts have gained popularity due to their adhesive capacity and ability to form a gap-free single unit. Prefabricated posts can be made of zirconia, glass, quartz or polyethylene fibers [2]. To select an appropriate material for prefabricated posts, fracture resistance should be considered as an important physical property. Carbon and glass fiber posts are favorable in this manner [3]. Satisfactory sur-vival rates have been reported in teeth that have been restored with fiber reinforced composite (FRC), posts made of quartz or glass fibers embedded in a matrix of epoxy or methacrylate resin [4].
However, considerations for appropriate post selection should not be limited to fracture resistance and must include efficiency of post removal. In several incidences, post removal and endodontic retreatment is required due to unsatisfactory post length, diameter, or apical seal [5]. Different techniques for post removal are selected based on post material and device availability, including burs to drill, ultrasonic vibration, and solvents used with endodontic files [6]. These methods can be relatively challenging, increase the risk of fracture [7], and cause pain and discomfort to the patient [8].
Since their introduction in the 1990s, lasers have revolutionized modern dental practice. Lasers can be applied for several purposes, such as diagnostic applications (caries detection), tooth whitening, resin curing, and cavity preparation [9]. Erbium: yttrium aluminum garnet (Er: YAG) laser, is readily used for cutting enamel, dentin, and gingival depigmentation procedures [10], due to its bactericidal effects and ability to cause minimal pain [11]. Moreover, Er: YAG has been used in the removal of glass ceramic brackets and porcelain veneers [12][13][14]. Namely, Oztoprak et al. [12] reported the effectiveness of Er: YAG laser on thermal softening and degradation of adhesives, resulting in debonding of ceramic brackets. Moreover, Morford et al. [14] conducted a study on IPS Empress and IPS e.max porcelain veneers, concluding that the application of Er: YAG laser can be effective in debonding and preserving tooth structure. Despite these developments, the debonding properties of Er: YAG laser on different post materials remains largely unexplored. Therefore, in this study, the effects of Er: YAG laser on debonding cemented glass fiber posts in endodontically treated teeth is investigated. The null hypothesis was that laser irradiation could not reduce the bond strength of glass fiber posts.

Materials and Method
A minimum sample number of 36 (power of 80%) was calculated. However, for higher accuracy, 40 samples were recruited and consented for the study. Samples included single-rooted mandibular premolars with the same length, extracted due to periodontal, prosthetic or orthodontic reasons, which were divided into two groups (20 in each group).
For disinfection, each tooth was stored in 5% chloramine T (Mina Tajhiz-co, Tehran, Iran) for 48 hours, before placing in distilled water, which was changed every day until examination. Soft tissue and dental plaques were removed from the root surface using periodontal scaler. Two radiographs were taken to assess the anatomic structure. Any tooth with internal/external resorption, two or more roots or canals, calcification, fracture lines and cracks detected by stereomicroscope was excluded from the study. Each tooth was sectioned from the crown with a water-cooled diamond disk (Buehler, Lake Bluff, Illinois, USA) to reach a standard root height of 15 mm.  [16] to prevent extreme heat generation and decrease crack formation. A pilot study was conducted to determine the best parameters for laser irradiation. Six extra samples were randomly selected for the pilot study. Table 1 presents the tested parameters. According to the results of the pilot study, the best effects were obtained using 7 W, interrupted pulse duration and discontinued rinsing spray. For the main study, 40 samples were divided into two groups by simple randomization method (using a hexagonal dice). The first group received no laser and the second group was irradiated with the laser. The laser tip was placed on the coronal part, exactly on the fiber post, at the cut site.
Push-out test was used to evaluate tensile resistance.   where R is the coronal diameter of the post, r is the apical diameter of the post and h is the slice thickness in mm.
Five samples from each group were randomly selected, which were prepared for scanning electron microscopy (SEM) analysis. Samples were coated with gold and evaluated using SEM (Zeiss Evo 50; Carl Zeiss, Oberkochen, Germany) at a magnification of ×100. Finally, the fracture mode was reported as cohesive fracture in post, cements, dentin, adhesive failure between cement and post, adhesive failure between cement and dentin, and mixed.

Statistical analysis
The effects of laser irradiation and location on bond strength were evaluated using two-way ANOVA. As the interaction effect was significant, subgroup analysis was applied. Independent sample t test was used to compare lasered and non-lasered groups in each location and one -way ANOVA and Tukey's HSD post hoc test were used to compare the strength between locations in each group.

Results
In this study, 20 teeth in each group and 3 cuts on each tooth resulted in 120 samples. Tensile strength measurements are presented in   The comparison of distribution of fractures at the five sites between the laser-irradiated and the control group showed that the highest number of fractures occurred at cement-dentin interface. Three fractures were occurred in the cement-dentin interface. One fracture was mixed and one was cohesive in the post.

Discussion
In the present study, we evaluated the effect of Er: YAG laser on the debonding of glass fiber posts cemented using resin cement. The findings showed that Er: YAG laser effectively reduced bond strength of the glass fiber post, rejecting the null hypothesis.
The pilot study showed that maximum bond strength reduction was obtained by employing the following parameters: 7 W, 350 mJ, pulse length of1 min duration, frequency of 20 Hz, and discontinued rinsing spray. In the main study, although complete debonding was not achieved, the results showed significant bond strength reduction in the laser-irradiated group. Moreover, the push-out test results showed a significantly higher tensile resistance in the control group than the laserirradiated group.
To the author's knowledge, just one study has been reported the efficacy of Er: YAG laser on debonding glass fiber posts using resin cement [19]. This study reported that the advantage of using laser instead of ultrasonic to retrieve glass fiber posts is causing lower temperature at the root surface and the post removal can be 5 times faster by irradiation of Er:YAG [19]. These results are in accordance with the results of the present study. Different studies have suggested that Er: YAG laser is efficient in removing composite restorations and debonding ceramic brackets [12][13][14]. In a bovine study, irradiation of Er: YAG (4.2W for 9 seconds) using the scanning method successfully enabled debonding polycrystalline ceramic brackets with lower shear bond strengths and higher adhesive remnant index [12].
Moreover, Mondethu et al. [13] investigated the effect of Er: YAG (600 mJ per pulse, 800 μs duration) on debonding of ceramic orthodontic brackets and reported successful debonding in 95% of the samples without damaging the enamel.
Differences between the results of the present study and the above-mentioned studies can be due to the study design; explicitly, this study was in vitro, while both the above-mentioned studies were investigated in vivo [12][13][14]. Additionally, laser parameters vary greatly among the different studies, which could justify different results [20].
Er: YAG laser is effective in debonding porcelain veneers [14] and ceramic restorations [21] with thermal and photo ablation of the resin cement [21]. In photo-  In the present study, post spaces were pretreated with chlorhexidine, which have been reported to increase bonding strength [27][28]. Different factors can play a role in the debonding efficacy of lasers, such as the type of rinsing material [29][30], resin cement [21,31] and root canal sealer [32] used. Aging can also affect bonding strength [33], although this was not evaluated in this study.
According to the fracture site, the highest fracture frequency was observed between the resin-dentin inter- has not been evaluated, and energy absorbing factors using techniques such as Fourier-transformed infrared spectroscopy [14] has not been investigated. In addition, the possible effects of laser irradiation on the periodontal ligament have not been considered. Therefore, further studies are required before Er: YAG laser can be verified as a bond-reducing removal method in clinical settings.

Conclusion
According to the results of the present study, the most effective Er: YAG laser parameters for reducing bond strength was irradiation for 1 min, at 7 W, 350 mJ and frequency of 20 Hz, and discontinued washing sprays.
Using these parameters, the findings showed that while complete debonding was not achieved, Er: YAG could effectively reduce bonding integrity and tensile resistance of glass fiber posts. These findings highlight the potential effect of Er: YAG laser irradiation in facilitat-ing the removal of glass fiber posts cemented in root canaled premolar teeth with resin cement.